The present invention relates in general to an apparatus and method of forming a continuous layer of thermoplastic material, and more particularly, to such an apparatus and method which enhances film quality and formation by eliminating localized film distortion and non-uniform heat dissipation during the film casting process.
In a film casting process, a thin sheet of molten thermoplastic material is extruded from a die and directed onto a continuously rotating casting surface, such as the surface of a highly polished metal cylinder or roll. During the casting process, the opening of the extrusion die is arranged such that the transverse length of the molten thermoplastic material and the longitudinal center line of the casting cylinder are parallel to one another. During this stage of the casting process, air or other gases should be prevented from entering and subsequently being trapped between the cast film and the surface of the rotating casting cylinder. In this regard, it is known that a rotating cylinder will entrain a layer of air along its outer surface. Ultimately, trapped air between the cast film and the surface of the rotating cylinder will result in localized film distortion and non-uniform heat dissipation during solidification of the film.
U.S. Pat. No. 3,154,608 has recognized that if too much air or smoke from molten polyolefin material becomes trapped between the surface of the casting cylinder and the polyolefin film, the optical properties of the resulting film are adversely effected. The adverse effects resulting from trapped air or smoke is avoided by providing a vacuum chamber or suction box located between the extrusion die and the moving surface of the casting cylinder. By maintaining a partial vacuum within this region, there is minimized the trapping of air between the extruded polyolefin material and the casting surface which would otherwise impair optical properties of the formed film. The creation of a partial vacuum is also disclosed to have the effect of withdrawing smoke generated by the hot extruded polyolefin material which, if trapped between the material and the casting surface, would also impair optical properties of the resulting film. In addition to these vacuum devices being relatively expensive, it is known that the removal of an entrained air layer by means of such a device necessitates that the vacuum level be adjusted accurately to prevent a potentially high vacuum from inducing the molten polyolefin film to bend backwards towards the vacuum device and, therefore, adversely effecting its uniform formation and resulting film qualities.
Attempts have been made in the paper making industry to employ other devices to cause a web of paper to contact a greater portion of the circumferential surface of a cylinder or the planar surface of a guide table. For example, there is known from U.S. Pat. No. 3,733,711 the use of one or more air foils on one side or the other of a web to create either a localized positive or negative pressure. In the case of a positive pressure, the web is forced into greater contact with the surface of the cylinder, while in the case of a negative pressure, the web is sucked against a corresponding greater portion of the cylinder. In either case, there has been no attempt to eliminate the layer of entrained air which results from rotation of the cylinder. Similarly, there is known from U.S. Pat. No. 4,034,905 the use of an air foil to create a vacuum between a continuously moving web and an underlying guide table to allow web supply over a relatively long distance, at increased speed, without vibration and undesirable flutter of the web.
However, in these known apparatuses, air foils are relatively large and accordingly difficult to position in a region of limited space or one occupied by other devices. In the case of a film casting process, unlike that in a paper making process, this region is also occupied by a relatively large extrusion die, typically positioned from about 3 to 20 inches from the casting cylinder. Accordingly, there is an unsolved need for a device which is operable to deflect the layer of entrained air from the surface of a casting cylinder, and which can be easily positioned within a region of limited space, and in particular, one which includes an extrusion die for forming a continuous layer or film of thermoplastic material.